1. Field of the Invention
The present invention relates to a reticle and an exposure monitoring method, which are configured to monitor an effective exposure dose in a multiple exposure process. Moreover, the present invention relates to an exposure method and a manufacturing method for a semiconductor device, which apply the exposure monitoring method.
2. Description of the Related Art
A multiple exposure which required a plurality of exposure processes is attracting attention in recent years as a microfabrication technique for a semiconductor device. The performance of a semiconductor device is largely controlled by dimensions of wiring patterns. For this reason, a control parameter for enhancing dimensional accuracy in a process using the multiple exposure needs to be highly accurately controlled. When using the multiple exposure, an exposure condition is normally determined for each exposure step so that the multiple exposure is implemented by executing the respective exposure steps under the determined exposure conditions.
A photolithography is a process to transfer a circuit pattern on a semiconductor substrate coated with a resist film by use of an aligner. In pattern formation using a reduction projection aligner, resolution of the aligner is in proportion to a wavelength λ of exposure light and is in inverse proportion to a numerical aperture NA. Therefore, because of the demand for miniaturization of semiconductor devices, process improvement associated therewith has been heretofore conducted, including shortening of the exposure light wavelength and achieving a higher NA of a projection lens. However, along with the recent demands for further miniaturization of the semiconductor device, it has become extremely difficult to ensure appropriate exposure latitude and depth of focus. Thus, in order to make measurable improvements in the dimensional precision in processing by effectively utilizing a small exposure margin without causing a reduction in yield, i.e., the amount of production, an exposure dose and focus control with a higher degree of accuracy is required.
Proposals have been made for an exposure monitoring method with attention on exposure control, in “A. Starikov, “Exposure monitor structure”, SPIE Integrated Circuit Metrology, Inspection, and Process Control IV, vol. 1261, 1990, p. 315 and in Japanese Patent Laid-Open No. 2000-310850. The proposals have a common characteristic in that, in a stepper, an image is transferred with an inclined exposure distribution by using a reticle having a pattern therein with a dimensional ratio (a duty ratio) of a transparent portion and an opaque portion continuously changed in one direction by a pitch that cannot resolve images on the semiconductor substrate. By use of this method, a variation distribution of an effective optimum exposure dose in resist mask pattern formation can be provided. Meanwhile, there is also a known method of forming patterns provided with an inclined distribution of irradiation by means of continuously disposing a plurality of patterns having different transmittances (see “SPIE Optical Microlithography IX”, Vol. 2726, 1996, p. 799).
As described above, it is important in microfabrication to control exposure conditions for photolithography with high accuracy to obtain processing accuracy and uniformity in pattern dimensions of a semiconductor device. However, in the case of double exposure, there is a so-called “fogging” influence caused by a first exposure step and a second exposure step. As used herein, the term “fogging” refers to an affect in which an image is exposed by an undesirable light. The exposure dose is deviated from the exposure condition which is originally set up because of the fogging of the two exposure steps. As a result, the finished pattern dimensions are different from the original design. Moreover, although the current exposure dose monitoring method can estimate the effective exposure doses in the respective exposure steps in the multiple exposure, the exposure dose monitoring method cannot estimate the influences on the respective exposure doses which are caused by the multiple exposure process.